I regularly go 50+ Kadala’s without getting a single legendary. That’s “normal”. To get a set item that I need it easily takes a couple of hundred if she’s being stingy. And I’ve gone over 300 more than once. That’s the joy of RNG combined with bloated loot tables.
The math on this is pretty straightforward. There is a 10% chance of getting a legendary any time you gamble at Kadala’s shop. For something like shoulders there are 5 possible sets and the various legendary ones for your class, say 3 of them. That gives you a 1 in 80 chance of getting the shoulder you want and – more importantly – a 79 in 80 chance of getting something useless. After 100 tries that works out to 28% of not getting your item. After 200 tries it’s still around 8%. Given you’re trying to purchase 13 items that will likely happen on at least one of them. And may RNGeezus be with you if you trying to get a ring, weapon, or amulet that you need for a build. Gambling and upgrading rares are both miserable.